<!doctype html>
<html>
<head>
  <title>CodeMirror 2: Haskell mode</title>
  <link rel="stylesheet" href="../../lib/codemirror.css">
  <script src="../../lib/codemirror.js"></script>
  <script src="haskell.js"></script>
  <link rel="stylesheet" href="../../theme/elegant.css">
  <style type="text/css">.CodeMirror {
    border-top: 1px solid black;
    border-bottom: 1px solid black;
  }</style>
  <link rel="stylesheet" href="../../css/docs.css">
</head>
<body>
<h1>CodeMirror 2: Haskell mode</h1>

<form><textarea id="code" name="code">
  module UniquePerms (
  uniquePerms
  )
  where

  -- | Find all unique permutations of a list where there might be duplicates.
  uniquePerms :: (Eq a) => [a] -> [[a]]
  uniquePerms = permBag . makeBag

  -- | An unordered collection where duplicate values are allowed,
  -- but represented with a single value and a count.
  type Bag a = [(a, Int)]

  makeBag :: (Eq a) => [a] -> Bag a
  makeBag [] = []
  makeBag (a:as) = mix a $ makeBag as
  where
  mix a [] = [(a,1)]
  mix a (bn@(b,n):bs) | a == b = (b,n+1):bs
  | otherwise = bn : mix a bs

  permBag :: Bag a -> [[a]]
  permBag [] = [[]]
  permBag bs = concatMap (\(f,cs) -> map (f:) $ permBag cs) . oneOfEach $ bs
  where
  oneOfEach [] = []
  oneOfEach (an@(a,n):bs) =
  let bs' = if n == 1 then bs else (a,n-1):bs
  in (a,bs') : mapSnd (an:) (oneOfEach bs)

  apSnd f (a,b) = (a, f b)
  mapSnd = map . apSnd
</textarea></form>

<script>
  var editor = CodeMirror.fromTextArea(document.getElementById("code"), {
    lineNumbers:true,
    matchBrackets:true,
    theme:"elegant"
  });
</script>

<p><strong>MIME types defined:</strong> <code>text/x-haskell</code>.</p>
</body>
</html>
